This invention relates to key telephone intercom systems and, more particularly, to pulse selective or dial pulse key telephone intercom systems.
Prior key telephone intercom systems for dial pulse of dial selective telephone application detect off-hook condition of the calling station within the intercom system, receive subscriber station address information in the form of dial pulses transmitted along a common intercom circuit from the calling station, and provide selective access to and actuate the ring-out device associated with a called station within the system. Station address information is registered by a dial pulse counter which is reset following or in response to ring-out or end of dialing, as the case may be. The various timing functions incident to off-hook determination, dialing, ring-out and, in those applications involving more than ten intercom stations, multiple-digit transfer, are performed by timing devices which heretofore have been constituted by complex relay controls and/or multiple timing circuits.
One system of this type, disclosed in U.S. Pat. No. 3,909,551, employs three timers to effect basic dial selective intercommunication among a ten station intercom, and, in applications involving greater numbers of stations, a fourth timing circuit to provide the transfer timing function. In addition to excessive complexity imposed by the use of multiple timing circuits, this system can retain a station address if the calling station is returned prematurely to its on-hook condition before it is reset at completion of dialing, hence accessing the wrong subscriber station on the basis of false station address information during the next call sequence. This system also tends to be uneconomical from the standpoint of installation of servicing because of fault detection or replacement incident to relay failure in the folded relay trees associated with the pulse counters, failure of merely one relay causing the entire ring-out circuit to shut down.